prop pitch

[Lalli]

I have a 1969 mercury 1250, it runs very good. It is on a 1969 Larson Lapline Tri-Hull. It seems to me that it should be a little faster, I can get it up to about 27 mph and that is it. I installed a hydrofoil and that didn't seem to help. Now, my next question is if changing the prop will help. Right now it has a 21 pitch on it, I don't know if that is to much or not enough, or maybe i just have a slow boat. I'm not complaining, I just want to get the maximum performance that i can. I mostly use the boat for fishing, and would just like to get from spot to spot a little faster if possible. Thanks

 

[steelespike]

Re: prop pitch

Need more info.Boat detail ;size, approximate weight,present max rpm and speed by gps is best.Motor is rated 4,800-5300 rpm should operate at at least 5,300.Without rpm figures really can't tell much.Unless the boat is a real tub 27 seems slow.
You might confirm it is running on all 6.be sure the throttle opens all the way.
Antivent plate (just above the prop) should be at least even with the bottom of the boat. most usually end up a little above.

 

[180shabah]

Re: prop pitch

...and lose the foil, they don't increase speed, the reduce it.

 

[Lalli]

Re: prop pitch

The boat is 16 ft, the antivent plate is slightly above the bottom of the boat, the rpm's at wot are right around 5200. As for running on all six I just assume it is because it runs very smooth, no chugging or sputtering. I don't Know the exact weight of the boat because the I.d. plate is not on it, I know I need to get one put in there. I think the throttle is opening all the way if I am getting 5200 rpm's out of it. I did replace the floor last year and all the saturated foam that was under it, so weight from that should not be an issue. Thanks

 

[steelespike]

Re: prop pitch

There are some who can't tell when a 2 cylinder 2 stroke is running in one cylinder.
but for the poor performance.
So it wouldn't be surprising to not think a 6 is running on 5 except for subpar performance.If you feel your hole shot is ok your probably good.Your rpm seems ok.
I don't know your gear ratio but 27 seems slow for a 21" prop.Also seems slow for
a 125 on a 16 ft boat.Does the hull respond when you trim up?While I don't think the tri hull is the fastest on the water I think a healthy 125 should be pushing in the 40s.

 

[Lalli]

Re: prop pitch

I have manual trim on this boat so I have to stop the boat, pull the engine up, and set it where I want it. I think there are 6 different settings, I have experimented with all of them and found the position that gets me the most top end. As far as running on all 6, how do I investigate into that to find out, or should I take it into a mechanic to find out. Thanks for the replies

 

[hwsiii]

Re: prop pitch

Lalli, Spike is right as always. I show your motor to have a recommended WOT range of 4,800 to 5,300 RPM with a gear ratio of 2:1, please verify this. If my information on your gear ratio is correct then you have a Prop Slip of 48%, which is unbelievable.
In my opinion, your tachometer is wayyyyyy off, unless you have a LOT of barnacles or algae on the bottom of the boat. You might try turning the switch on the back of the tach 10 or 12 times to clean the contacts, and then bring a mechanics tach to verify the RPM.
I show you should be using about a 17" pitch prop with a top speed of about 36 MPH with my calculations.

Prop Slip

Also take pictures and tell us the Anti-ventilation Plate height above the bottom transom of the of boat, if it is an outboard in inches, use a board under the keel and sticking out to the anti ventilation plate for a reference

I would also recommend a 4 blade prop for that boat, as those older trihulls always seem to have a lot of drag and create high prop slip, but not anywhere near what you have.


H

 

[Lalli]

Re: prop pitch

I will take pics tomorrow of the antiventilation locaition. I can't say for sure, but it seems like the tach is fairly accurate because it sounds like the engine is running pretty fast at wot. Would the 17 pitch help my top speed that much as things are now? thanks

 

[hwsiii]

Re: prop pitch

Lalli, I can say for sure, if you are turning 5,200 RPM with 21" pitch right now you will be turning 6,400 RPM, with the same manufacturer and model of prop in a 17" pitch.
I find it impossible to believe you have a 48% prop slip if the bottom of your boat is not messed up, or the prop is not 21" pitch or the prop is messed up.

Try reading this for some help.

http://forums.iboats.com/showthread.php?t=365653



H

 

[a70eliminator]

Re: prop pitch

I always thought top speed was the product of RPM and final drive ratio, not horsepower.
If one motor cranks up 90hp and another 125 yet both achieve the same RPM than why would the 125 have a higher top speed? Doesn't it just mean the larger can reach top RPM faster and carry a heavier payload. Either way dropping from a 21p to a 17p is quite a difference, top speed will drop and you'll more than likely overwind the motor.

 

[180shabah]

Re: prop pitch

top speed is all about hp. given your example above of 90 hp vs 125 hp, there are still too many variables. if both boats are the same, as are the prop and gear ratio then yes, at 5500 rpms both boats will be traveling at the same speed.
HOWEVER - the throttle on the 125 will not be fully opened. In fact it will be closed to a point where the engine isonly producing........wait for it..........90hp, just like the other engine.

If both engines were proped/geared correctly, they both achieve 5500rpm at WOT, then at any given engine rpm the 125 will be traveling at a higher speed than the 90.

 

[hwsiii]

Re: prop pitch

On the same boat, with the same manufacturer and model of prop the 125 will always turn more RPM with the same diameter and pitch of prop. Shabah is 100% right, speed is proportional to HP, that is what Crouch's formula does for figuring out how fast a boat will go if you increase the HP.



H

 

[Lalli]

Re: prop pitch

So as it is right now, either my antiventilation plate is in the wrong spot, or my tach is incorrect and I am not reaching max rpm's, which would have to do with my throttle being adjusted. Does this sound correct?

 

[hwsiii]

Re: prop pitch

It is your TACOMETER.


H

 

[Lalli]

Re: prop pitch

So if it is my tach, would I have to do adjust my throttle cable to get higher rpm's? Thank You very much for all of your advice.

 

[trendsetter240]

Re: prop pitch

So if it is my tach, would I have to do adjust my throttle cable to get higher rpm's? Thank You very much for all of your advice.

No. You need to verify that your tach is working correctly. If it is not you need to replace it.

The suggestion here is that you are not actually reaching 5200 RPMs and the tach is giving you the wrong reading. And if that were the case, your motor is spinning at a much lower RPM and the prop you have has far too much pitch.

I agree that the 48% prop slip is highly unlikely. Could be that your tach is wrong or you forgot to pull up your anchor!